1. Field of the Invention
The present invention pertains to surgical cutting instruments and, more particularly, to surgical cutting instruments having relatively movable, elongate inner and outer tubular members for cutting tissue, with the inner and outer tubular members being mounted with minimal or no gap or clearance therebetween.
2. Description of the Prior Art have been proposed, and such instruments are usually elongated to permit distal ends of the instruments to be positioned at internal operative sites through one or more portals. Surgical cutting instruments for endoscopic use frequently include relatively movable, cooperating, concentric inner and outer tubular members that are longitudinally elongated such that distal ends of the inner and outer tubular members can be positioned at internal operative sites via the portals while proximal ends of the inner and outer tubular members can be secured, externally of the body, in a handpiece for relatively moving the inner and outer tubular members. The distal end of the outer tubular member is commonly formed with an opening defining a cutting port, or window, to receive bodily tissue, and the distal end of the inner tubular member is formed with a surface, or edge, to engage the tissue through the opening when the inner tubular member is moved relative to the outer tubular member. The surface or edge on the inner tubular member distal end cooperates with the opening in the outer tubular member distal end to shear, abrade or otherwise cut the tissue when the surface is moved adjacent the opening. In many cases, the inner tubular member is rotated in the outer tubular member by a motor disposed in the handpiece and coupled to the proximal end of the inner tubular member with the motor being controlled by switches on the handpiece, the floor, or a console powering the handpiece. The surface or edge on the inner tubular member distal end and the opening in the outer tubular member distal end are typically configured to produce a variety of cutting functions, such as whisker cutting, synovial resection, arthroplasty burring or abrading, side cutting, meniscus cutting, trimming, full radius resection and end cutting, appropriate for diverse types of tissue, such as soft tissue, cartilage and bone. Suction is normally produced through the handpiece and the lumen of the inner tubular member to permit cut tissue to be aspirated from the surgical site through the inner tubular member.
During operation, the surface or edge on the inner tubular member distal end must be accurately positioned and aligned relative to the opening in the outer tubular member distal end to insure a precise cut and optimum cutting efficiency as measured by the force required to rotate the cutting surface. In other words, the inner tubular member cannot be cocked or oriented askew with the longitudinal axis of the surgical cutting instrument when the cutting surface is moved through tissue positioned in the opening. Accordingly, a bearing structure is commonly necessary to maintain exact positioning and alignment of the cutting surface with the opening while allowing the inner tubular member to rotate freely within the outer tubular member; and, in most instances, the bearing structure is disposed at the inner tubular member distal end or at spaced locations along the length of the inner tubular member. Such bearing structure possesses many disadvantages including structural complexity, increased manufacturing cost resulting in a very expensive product that is incompatible with disposability, or single patient use, and reduced reliability and anticipated life due to greater opportunities for malfunction from sticking and obstruction, and a simplified bearing for use in surgical instruments formed of relatively movable, inner and outer tubular members to allow the instruments to safely withstand increased radial or side loads has not been proposed.
Another approach to obtaining accurate alignment and positioning of a movable inner tubular member within an outer tubular member of a surgical instrument involves reducing the gap, or clearance, between the outer diameter surface of the inner tubular member and the inner diameter surface of the outer tubular member such that there is minimal or virtually no gap or clearance between the inner and outer tubular members. By making the outer diameter of the inner tubular member substantially the same as the inner diameter of the outer tubular member, or as close as possible thereto, misalignment of the inner tubular member within the outer tubular member in response to the cutting action can be avoided. A major drawback to this approach is that it is preferred to construct the inner and outer tubular members of AISI 300 Series stainless steel due to the cost benefits obtained with stainless steel; however, the physical properties of 300 Series stainless steel limit the effectiveness of 300 Series stainless steel as a bearing surface. In particular, surgical instruments having relatively movable, inner and outer tubular members made from 300 Series stainless steel are prone to failure due to the relative softness of 300 Series stainless steel and the surface to surface contact that occurs between the inner and outer tubular members when the gap between the inner and outer members is very small. Additionally, great frictional heat is generated at areas of contact between the inner and outer tubular members due to the relatively high coefficient of friction for 300 Series stainless steel. The frictional heat generated during operation of a surgical cutting instrument can be significant as the inner tubular member can rotate within the outer tubular member at very high speeds, i.e. 2500 RPM and greater. When inner and outer tubular members made from 300 Series stainless steel bear on and contact each other, considerable frictional heat is produced at the contact areas causing the inner tubular member to thermally expand. As the inner tubular member expands, areas of contact between the inner and outer tubular members increase producing greater friction and wear due to abrasion, particularly at the distal end of the instrument where heat can become concentrated. Eventually, the grain structure of the stainless steel inner and outer tubular members begins to "flake", and increased abrasion and frictional heat occurs due to metal flaking. Further expansion of the inner tubular member creates localized hot spots at the contact areas, and welding of the inner and outer tubular members at the hot spots occurs. As the inner tubular member continues to rotate within the outer tubular member, additional weld junctions are produced eventually resulting in galling, wherein the weld junctions sever or tear creating a build-up of metal on one of the tubular members that promotes abrasion and thermal expansion until the inner tubular member has expanded further than the gap between it and the outer tubular member and the instrument seizes. Biocompatible lubricants for effectively reducing abrasion between relatively movable inner and outer tubular members of surgical cutting instruments have not previously been recognized or proposed; and, accordingly, wear or abrasion, metal flaking, galling and catastrophic seizure in surgical instruments having relatively movable inner and outer tubular members made from stainless steel have commonly been avoided by increasing the gap or clearance between the inner and outer tubular members such that contact between the inner and outer tubular members is prevented or greatly minimized. Although increasing the clearance between the inner and outer tubular members can prolong the life of surgical cutting instruments, cutting quality and efficiency are drastically reduced, and the instruments perform poorly in surgical procedures requiring precision cutting. A relatively large gap or clearance between the inner and outer tubular members permits misalignment of the inner tubular member relative to the longitudinal axis of the surgical instrument in response to the cutting action. Accordingly, the cutting surface on the inner tubular member will not be properly aligned with the opening in the outer tubular member resulting in cutting inaccuracies and inefficiencies. Furthermore, a relatively large gap between the distal end of the inner tubular member and the distal end of the outer tubular member allows tissue in the opening to become caught between the inner and outer tubular members during operation; and, in many instances, tissue can become wrapped around the inner tubular member as it rotates within the outer tubular member thusly compromising the cutting procedure. Moreover, elongated surgical instruments are typically subjected to considerable thrust and radial loads during operation, particularly at the distal ends of the inner and outer members, and inner and outer tubular members made from 300 Series stainless steel cannot withstand high thrust and radial loads without experiencing deformation and damage.
A surgical cutting instrument having an inner tubular member coated with titanium nitride and movably mounted in an outer tubular member has been proposed in U.S. Pat. No. 4,923,441 to Shuler. The titanium nitride coating extends continuously along the outer diameter of the inner tubular member from a proximal end to a distal end including a cutting surface at the distal end. Forming one of the inner and outer members of gall-resistant material to provide superior bearing and strength characteristics has not been recognized. Furthermore, the use of a chromium coating or a surface hardened case as a bearing surface on the outer diameter of a stainless steel inner tubular member has not been recognized or proposed.
Chromium coatings have been utilized in industrial applications on metal surfaces, and prior uses of chromium coatings are illustrated in U.S. Pat. Nos. 1,753,773 to Champion; 1,809,412 to DeVore; 3,063,763 to Zubrisky; 4,335,924 to McCloskey; and 4,692,080 to Hyner et al. Chromium coatings have not been utilized as a bearing surface on an inner tubular member of a surgical cutting instrument formed of relatively movable, elongate inner and outer tubular members to allow the outer diameter of the inner tubular member to be the same as, or as close as possible to, the inner diameter of the outer tubular member to eliminate abrasion or wear, metal flaking and galling or seizure in the surgical instrument and to permit the inner and outer tubular members to withstand increased thrust and radial roads while being made from 300 Series stainless steel.
Gall-resistant materials have also been employed in industrial applications, and illustrative gall-resistant stainless steel alloys include Nitronic 60, manufactured by Armco Steel Corporation and disclosed in U.S. Pat. No. 3,912,503 to Schumacher et al, Waukesha 88, made by Waukesha Foundry, Inc. and Gall-Tough Stainless, manufactured by Carpenter Technology Corporation. It has not been recognized to form an inner or outer tubular member of a surgical cutting instrument including relatively movable, elongate inner and outer tubular members from gall-resistant material to allow the inner tubular member to be mounted in the outer tubular member with little or no gap therebetween while permitting the surgical instrument to resist wear and abrasion, metal flaking, galling and seizure and to withstand high thrust and radial loads.
Surface hardening of stainless steel has also been employed in industrial applications to increase the operational life of stainless steel components; however, it has not been recognized to surface harden an outer diameter surface of an inner tubular member of a surgical cutting instrument formed of relatively movable, inner and outer tubular members to provide a bearing surface along the inner tubular member allowing the inner tubular member to be mounted within the outer tubular member with little or no gap therebetween while avoiding wear or abrasion, galling and seizure of the surgical instrument and while allowing the surgical instrument to withstand high thrust and radial loads.